Conventionally, blending an inorganic filler powder into a resin has been performed for the purpose of controlling the coefficient of thermal expansion or other purposes. For example, in a multi-layer printed circuit board in which a resin is used, the coefficient of thermal expansion of an insulating layer needs to be decreased because a difference in coefficient of thermal expansion between a conductor layer and the insulating layer promotes the occurrence of cracks. For this reason, an inorganic filler powder, such as silica powder, is blended into a resin. Silica powder is widely used as an inorganic filler powder because it is excellent in physical strength and thermal resistance (see, for example, Patent Literature 1).